1. Field of the Invention
The present invention relates to a non-stick coating system which is applied in multiple coats to the surface of a substrate to thereby provide a coated substrate having a non-stick coating to which extraneous materials will not adhere. The non-stick coating includes a primer, a topcoat, and optionally, a midcoat. More particularly, the present invention relates to a multiple coat composition including a primer which provides excellent substrate adhesion and intercoat adhesion, and also provides excellent intracoat cohesion between the binder and fluoropolymer components thereof.
2. Description of the Related Art
The use of non-stick coating systems which are applied to a substrate in multiples layers has been known for more than 20 years. Typically, these coating systems include two layers consisting of a specially formulated primer and topcoat, but systems incorporating one or more intermediate midcoats are also known. The primers for such systems typically contain a heat resistant organic binder resin and one or more fluoropolymer resins, along with various opaque pigments and fillers. The midcoats contain mainly fluoropolymers with some amounts of opaque pigments, fillers and coalescing aids, while the topcoats are almost entirely composed of fluoropolymers. In such systems, the binder resin of the primer adheres to the substrate, while the fluoropolymer adheres to subsequent midcoat and/or topcoat layers. The binder and fluoropolymer of the primer are attached to one another via an essentially mechanical bond resulting from the mixing of the two components, followed by the curing of the primer after application to a substrate. An early example of such a system is found in U.S. Pat. No. 4,049,863 to Vassiliou (1977).
Over the course of the development of such coating systems, it has come to be accepted that the adhesion of the primers to metal substrates is a function of the amount of organic binder resin present at the interface between the substrate and the coating. It is well known that organic binders, such as polyamideimide, polyether sulfone and polyphenylene sulfide, for example, have excellent adhesion to metals.
Likewise, it has come to be accepted that the adhesion of subsequent layers or coats to the primer depends upon the amount of fluoropolymer present at the interface between the primer and the next layer. This is due to the fact that adhesion between the primer and subsequent layers is achieved by the fusing or sintering of the fluoropolymer in the primer to the fluoropolymer in the subsequent layer during the curing process at temperatures above the melting point of the fluoropolymers.
A mixture of ingredients is expected, however, at both the primer-substrate and the primer topcoat interfaces, because the primers themselves include a mixture of ingredients. Some fluoropolymer will therefore be present at the primer-substrate interface, and thereby adversely affect adhesion to the substrate, and some binder will be present at the primer-topcoat interface, and thereby adversely affect the intercoat adhesion with subsequent layers. For these reasons, much development effort has been directed to achieving a greater degree of separation or stratification between the binder resin and the fluoropolymer in the primer, so that the binder moves to the bottom of the primer layer to increase substrate adhesion, and the fluoropolymer moves to the top of the primer layer to increase intercoat adhesion. Such differences in composition from top to bottom of the primer layer are known as “concentration gradients,” and are described in U.S. Pat. No. 4,087,394 to Concannon (1978) and in U.S. Pat. No. 5,240,775 to Tannenbaum (1993).
The development of sharper concentration gradients in primers has become more important with the increased emphasis on applying non-stick multi-layered coatings to “smooth” substrates. Traditionally, metal substrates had been roughened by gritblasting or finishing (sanding) before application of a non-stick coating thereto. Application to “smooth” substrates eliminates the slow and expensive roughening process. For purposes of the present application, a “smooth” substrate refers to a substrate that has been chemically cleaned and lightly etched to an average roughness (Ra) of less than 100 microinches (2.5 microns), and preferably of less than 50 microinches (1.25 micron). By way of comparison, untreated rolled aluminum, for example, has an average roughness of 12-20 microinches (0.25-0.50 microns), and gritblasted aluminum has an average roughness of 160 to 220 microinches (4 to 5.25 microns).
A more recent direction in the development of primers is the inclusion of hard fillers to increase scratch and damage resistance. The use of such fillers is well known, and is also disclosed in U.S. Pat. No.4,049,863 to Vassiliou (1977), noted above, in a range up to 20% by weight of the solids. More recent disclosures of the use of hard fillers may be found in U.S. Pat. No. 5,250,356 to Batzer (1993) and U.S. Pat. No. 5,562,991 to Tannenbaum (1996), wherein the amount of filler used is as high as 35% by weight of the solid material.
In practice, the main raw materials used in primers for non-stick, multi-layered coatings have not changed greatly in recent years. The preferred organic binder remains a polyamideimide (“PAI”) resin, prepared as an aqueous polyamic acid salt. The preferred fluoropolymer remains an aqueous dispersion of polytetrafluoroethylene (“PTFE”) resin with a melt viscosity greater than 103 poise. The recent improvements in the performance of primers has come from (1) incrementally enhancing the original, basic mechanism for promoting substrate adhesion and intercoat adhesion, viz., increasing the stratification of the primer through the addition of fluoropolymers such as fluorinated ethylene propylene (“FEP”) that have lower melt viscosity and lower melting points than PTFE, and (2) adding ever increasing amounts of hard fillers for improved penetration and scratch resistance.
Adding hard fillers, while increasing the damage resistance of the primer, has the undesirable effect of adding an additional ingredient to the primer, which competes for space at the critical primer-substrate and primer-topcoat interfaces, and thereby detracts from the amount of preferred materials, as described above, which are required at the primer-substrate and primer-topcoat interfaces for maximum bonding.
Additionally, recent effort has been directed to the development of a single coat non-stick coating system having fluoropolymer and binder components, in which stratification between the fluoropolymer and binder is allowed in order to enhance the substrate adhesion and the release properties of the coating without failure between the fluoropolymer and binder components, and also, which coating has excellent adhesion to smooth substrates and allows the incorporation of high levels of fillers thereinto for improved scratch and damage resistance.
As shown in FIG. 1, compensating for this loss of bond strength by forcing greater separation/stratification of these materials has an obvious limit. In coating system 10, the stratification between binder 12 and fluoropolymer 14 of primer 22 allows binder 12 and fluoropolymer 14 to adhere well to substrate 16 and to topcoat 24, respectively. However, the large degree of stratification between binder 12 and fluoropolymer 14 actually creates, in effect, a new binder-fluoropolymer interface 20. Specifically, the mechanical interconnection between binder 12 and fluoropolymer 14 is minimal, and subject to failure, at binder-fluoropolymer interface 20. Therefore, failure of coating system 10 occurs within primer 22 itself, because the connection between the organic binder and fluoropolymer (the intracoat cohesion) is a mechanical interconnection, and prone to separation failure upon a large degree of stratification within primer 22. The same type of failure illustrated in FIG. 1 may occur in conventional single coat, non-stick coating systems which include fluoropolymer and binder components.
What is needed is a multilayer non-stick coating system, the coating system including a primer which exhibits both excellent primer-substrate adhesion and primer-topcoat adhesion, yet is resistant to separation failure at the interface between the binder and fluoropolymer components of the primer.
A further need is for a multilayer non-stick coating system, including a primer which allows for the incorporation into the primer composition of a high level of fillers to provide increased damage resistance.
A still further need is for a multilayer non-stick coating system, including a primer which has excellent adhesion to a smooth substrate.